M&S says

When looking for support there’s no need to compromise on style. With our medium support tights, your legs are sure to look and feel wonderful. These comfortable tights improve circulation in your legs and reduce ankle tension, for the active wearer who needs a touch of added support. Reinforced toes and added stretch further enhance the comfort and durability of these 15 denier sheer tights, perfect for travel or long days on your feet.